The present invention relates to a method for dust-proofing in the manufacturing process of electronic devices, such as semiconductor devices, e.g., LSIs and ULSIs, and liquid-crystal display panels and the like, and also relates to a pellicle used therefor. In particular, the invention relates to a method for dust-proofing of a photomask in the photolithographic processing of an electronic device by use of light having a wavelength of 500 nm or shorter by covering the photomask with a pellicle having excellent transparency in the wavelength region.
In the manufacture of semiconductor devices, such as LSIs and ULSIs, and liquid-crystal display panels and the like, the method of photolithography is widely undertaken for patterning on a semiconductor silicon wafer or a liquid-crystal master plate by patternwise exposure of the photoresist layer to light through a photomask bearing the desired pattern. When the photomask used here carries dust particles deposited thereon, the light beam is unevenly absorbed or scattered by the dust particles so that the pattern formed on the silicon wafer and the like is not a high-fidelity reproduction of the original pattern or the edge lines of the patterned areas may be rugged without smoothness sometimes with a decrease in the contrast of the patterned images resulting in a great decrease in the quality of the finished products.
Although the above mentioned works are usually performed in a dust-free clean room, it is an extremely difficult matter to keep the photomask in a perfectly dust-free condition throughout the procedure. A generally undertaken countermeasure in this regard is to cover the photomask with a transparent pellicle applied thereto. Various polymeric materials have been proposed and are under practical use as a material of such a pellicle including nitrocellulose, cellulose acetate and the like (see, for example, Japanese Patent Kokai 60-75835).
These conventional polymeric materials for pellicle have several disadvantages. For example, pellicles made from nitrocellulose or cellulose acetate have a relatively strong absorption band of light in the short wavelength region of 210 to 500 nm and they are relatively unstable to cause yellowing in the lapse of time so that such a pellicle can hardly be used in the photolithography for the manufacture of ULSIs or ultra-high density liquid-crystal display panels where it is essential to conduct the patterning by using light of an as short as possible wavelength in the ultraviolet region such as the light beams emitted from an excimer laser or so-called g-lines and i-lines.
The inventors have previously proposed use of a pellicle made from a polymer of vinyl trimethyl silane or a pullulan compound in place of nitrocellulose and cellulose acetate in Japanese Patent Kokai 2-5861 and 2-230254, respectively. The pellicles made from these polymeric materials have excellent light transmissivity even in the wavelength region of 210 to 500 nm and sufficiently stable against yellowing in the lapse of time but they are disadvantageous in respect of the poor mechanical strengths as compared with nitrocellulose so that no quite satisfactory dust-proofing effect can be obtained therewith.